Phase rotator calibration of a multichannel radar transmitter

ABSTRACT

Multi-channel radio frequency (RF) transmitter ( 100 ) and method of calibrating the transmitter are provided. In an embodiment, the method involves applying an intermediate frequency (IF) signal with different compensation values on a first phase rotator ( 128 ) in a first channel transmitter module (TX 1 ) of the transmitter, wherein the different compensation values are designed to compensate for a particular phase influencing factor, applying phase codes with the same different compensation values for different phases on a second phase rotator ( 132 ) in a second channel transmitter module (TX 2 ), measuring resultant phase errors due to phase errors of the first and second channel transmitter modules for the different compensation values, and based on the resultant phase errors, selecting one of the different compensation values to be used as a calibrated compensation value for the first and second phase rotators in the first and second channel transmitter modules to compensate for the particular phase influencing factor.

Embodiments of the invention relate generally to radar system and, moreparticularly, to radar transmitters with phase rotator calibrationcapabilities.

Improving the radar is a key aspect for the next generationFrequency-Modulated Continuous-Wave (FMCW) imaging radar systems. Theimprovement of the range detection can be addressed by using amulti-chip transceiver using beam steering techniques. It consists indynamically adapting the beam pattern of the antennas by changing thesignal phase in real time without changing the antenna elements or otherhardware. The beam steering performance depends on the accuracy of thecontrol of the phase of each antenna. Phase and amplitude of theradiation pattern are digitally controlled through a phase shifter. Anydistortions from the programmed phase and amplitude will degrade theradar performance.

SUMMARY

Multi-channel radio frequency (RF) transmitter and method of calibratingthe transmitter are provided. In an embodiment, the method involvesapplying an intermediate frequency (IF) signal with differentcompensation values on a first phase rotator in a first channeltransmitter module of the transmitter, wherein the differentcompensation values are designed to compensate for a particular phaseinfluencing factor, applying phase codes with the same differentcompensation values for different phases on a second phase rotator in asecond channel transmitter module of the multi-channel RF transmitter,measuring resultant phase errors due to phase errors of the first andsecond channel transmitter modules for the different compensationvalues, and selecting one of the different compensation values to beused as a calibrated compensation value for the first phase rotator inthe first channel transmitter module and for the second phase rotator inthe second channel transmitter module to compensate for the particularphase influencing factor.

In an embodiment, the different compensation values include DC offsetvalues to compensate for local oscillator to RF leakage, gain values tocompensate for gain imbalance, or phase values to compensate for phaseimbalance.

In an embodiment, the method further comprises applying a complex I/Qsignal to the first and second phase rotators from an I/Q coupler modulethat receives a local oscillator signal.

In an embodiment, measuring the resultant phase error includes mixingthe outputs of the first and second channel transmitter modules togenerate a mixed signal and performing fast Fourier transform (FFT) onthe mixed signal to measure the resultant phase error.

In an embodiment, measuring the resultant phase error further includesusing arctangent function on results of the FFT to measure the resultantphase error.

In an embodiment, the method further comprises converting the signal andthe phase codes from digital to analog using multiple digital-to-analogconverters prior to applying the signal and the phase codes to the firstand second phase rotators.

In an embodiment, selecting one of the different compensation valuesincludes summing resultant phase errors for the different compensationvalues to produce summed resultant phase error values corresponding tothe different compensation values and selecting the minimum value fromthe summed resultant phase error values to select the calibratedcompensation value.

In an embodiment, the method further comprises applying a particularphase code with the calibrated compensation value on the first phaserotator in the first channel transmitter module and the second phaserotator in the second channel transmitter module to produce outputsignals from the first and second channel transmitter modules,determining a phase misalignment between the first and second channeltransmitter modules using the output signals, and providing a phaseoffset to compensate for the phase misalignment so that signals to thefirst and second channel transmitter modules are modified to phase alignthe first and second channel transmitter modules.

In an embodiment, the method further comprises applying the signal withthe selected compensation value on the first phase rotator in the firstchannel transmitter module, applying the phase codes with the selectedcompensation value on the second phase rotator in the second channeltransmitter module, measuring phase differences between outputs of thefirst and second channel transmitter modules, comparing the phasedifference to at least one programmed threshold, and generating a flagto indicate phase drift when at least one of the phase differences isgreater than the at least one programmed threshold.

A multi-channel radio frequency (RF) transmitter in accordance with anembodiment of the invention comprises a first channel transmitter modulecomprising a first phase rotator, a second channel transmitter modulecomprising a second phase rotator, a mixer to combine outputs of thefirst and second channel transmitter modules, and a digital processingsystem operatively connected to the first and second channel transmittermodules and the mixer. The digital processing system is configured toapply an intermediate frequency (IF) signal with different compensationvalues on the first phase rotator, wherein the different compensationvalues are designed to compensate for a particular phase influencingfactor, apply phase codes with the same different compensation valuesfor different phases on the second phase rotator, measure resultantphase errors due to phase errors of the first and second channel modulesfor the different compensation values, and based on the resultant phaseerrors, select one of the different compensation values to be used as acalibrated compensation value for the first phase rotator in the firstchannel transmitter module and for the second phase rotator in thesecond channel transmitter module to compensate for the particular phaseinfluencing factor.

In an embodiment, the different compensation values include DC offsetvalues to compensate for local oscillator to RF leakage, gain values tocompensate for gain imbalance, or phase values to compensate for phaseimbalance.

In an embodiment, the digital processing system is further configured toapply a complex I/Q signal to the first and second phase rotators froman I/Q coupler module that receives a local oscillator signal.

In an embodiment, the transmitter further includes an FFT module toperform FFT on a mixed signal from the mixer for a phase errormeasurement.

In an embodiment, the digital processing system is further configured touse arctangent function on results of the FFT to measure the resultantphase error.

In an embodiment, the transmitter further comprises a plurality ofdigital-to-analog converters to convert the signal and the phase codesfrom digital to analog prior to the signal and the phase codes beingapplied to the first and second phase rotators.

In an embodiment, the digital processing system is further configured tosum the resultant phase errors for the different compensation values toproduce summed resultant phase error values corresponding to thedifferent compensation values and select the minimum value from thesummed resultant phase error values to select the calibratedcompensation value.

In an embodiment, the digital processing system is further configured toapply a particular phase code with the calibrated compensation value onthe first phase rotator in the first channel transmitter module and thesecond phase rotator in the second channel transmitter module to produceoutput signals from the first and second channel transmitter modules,determine a phase misalignment between the first and second channeltransmitter modules using the output signals, and provide a phase offsetto compensate for the phase misalignment so that signals to the firstand second channel transmitter modules are modified to phase align thefirst and second channel transmitter modules.

In an embodiment, the digital processing system is further configured toapply the signal with the selected compensation value on the first phaserotator in the first channel transmitter module, apply the phase codeswith the selected compensation value on the second phase rotator in thesecond channel transmitter module, measure phase differences betweenoutputs of the first and second channel transmitter modules, compare thephase differences to at least one programmed threshold, and generate aflag to indicate phase drift when at least one of the phase differencesis greater than the at least one programmed threshold.

A method of calibrating a multi-channel radio frequency (RF) transmitterin accordance with an embodiment of the invention comprises applying anintermediate frequency (IF) signal with different first compensationvalues on a first phase rotator in a first channel transmitter module ofthe multi-channel RF transmitter, wherein the different firstcompensation values are designed to compensate for a first phaseinfluencing factor, applying phase codes with the same different firstcompensation values for different phases on a second phase rotator in asecond channel transmitter module of the multi-channel RF transmitter,measuring first resultant phase errors due to phase errors of the firstoutputs of the first and second channel transmitter modules for thedifferent first compensation values, based on the first resultant phaseerrors, selecting one of the different first compensation values to beused as a first calibrated compensation value for the first phaserotator in the first channel transmitter module and for the second phaserotator in the second channel transmitter module to compensate for thefirst phase influencing factor, applying the IF signal with the firstcalibrated compensation value and different second compensation valueson the first phase rotator in the first channel transmitter module,wherein the different second compensation values are designed tocompensate for a second phase influencing factor, applying the phasecodes with the first calibrated compensation value and the samedifferent second compensation values for different phases on the secondphase rotator in the second channel transmitter module, measuring secondresultant phase errors due to phase errors of the first and secondchannel transmitter modules for the different second compensationvalues, and based on the second resultant phase errors, selecting one ofthe different second compensation values to be used as a secondcalibrated compensation value for the first phase rotator in the firstchannel transmitter module and for the second phase rotator in thesecond channel transmitter module to compensate for the second phaseinfluencing factor.

In an embodiment, the first and second different compensation valuesinclude DC offset values to compensate for local oscillator to RFleakage, gain values to compensate for gain imbalance, or phase valuesto compensate for phase imbalance.

Other aspects and advantages of embodiments of the present inventionwill become apparent from the following detailed description, taken inconjunction with the accompanying drawings, depicted by way of exampleof the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a multi-channel radio frequency (RF)transmitter in accordance with an embodiment of the invention.

FIG. 2A illustrates the influence of phase rotator gain imbalance onphase error at a mixer of the multi-channel RF transmitter.

FIG. 2B illustrates the influence of phase rotator phase imbalance onphase error at the mixer of the multi-channel RF transmitter.

FIG. 2C illustrates the influence of phase rotator LO-to-RF leakage onphase error at the mixer of the multi-channel RF transmitter.

FIG. 2D illustrates the combined effect of the three influences shown inFIGS. 2A-2C.

FIG. 3 illustrates modified signals being used in the multi-channel RFtransmitter to compensate for phase rotator LO-to-RF leakage, phaserotator phase imbalance and phase rotator gain imbalance.

FIG. 4A illustrates an unmodified IF signal being applied to a phaserotator of a first channel module of the multi-channel RF transmitter toproduce an output signal with phase error.

FIG. 4B illustrates a modified IF signal being applied to the phaserotator of the first channel module of the multi-channel RF transmitterto cancel out the phase error in the output signal.

FIGS. 5A, 5B and 5C illustrate a three-step calibration processperformed by the multi-channel RF transmitter in accordance with anembodiment of the invention.

FIG. 6 is a graph showing phase errors for different phase compensationvalues during a phase imbalance calibration in accordance with anembodiment of the invention.

FIG. 7A shows phase errors due to gain imbalance for no calibration, forcalibration using 8-bit digital-to-analog converters (DACs) and forcalibration using 15-bit DACs in accordance with embodiments of theinvention.

FIG. 7B shows phase errors due to phase imbalance for no calibration,for calibration using 8-bit digital-to-analog converters (DACs) and forcalibration using 15-bit DACs in accordance with embodiments of theinvention.

FIG. 7C shows phase errors due to LO-to-RF leakage for no calibration,for calibration using 8-bit digital-to-analog converters (DACs) and forcalibration using 15-bit DACs in accordance with embodiments of theinvention.

FIG. 8 illustrates a phase alignment process to phase align between thefirst and second channel modules of the multi-channel RF transmitter inaccordance with an embodiment of the invention.

FIG. 9 is a process flow diagram of a method of calibrating amulti-channel RF transmitter in accordance with an embodiment of theinvention.

Throughout the description, similar reference numbers may be used toidentify similar elements.

DETAILED DESCRIPTION

It will be readily understood that the components of the embodiments asgenerally described herein and illustrated in the appended figures couldbe arranged and designed in a wide variety of different configurations.Thus, the following detailed description of various embodiments, asrepresented in the figures, is not intended to limit the scope of thepresent disclosure, but is merely representative of various embodiments.While the various aspects of the embodiments are presented in drawings,the drawings are not necessarily drawn to scale unless specificallyindicated.

The described embodiments are to be considered in all respects only asillustrative and not restrictive. The scope of the invention is,therefore, indicated by the appended claims rather than by this detaileddescription. All changes which come within the meaning and range ofequivalency of the claims are to be embraced within their scope.

Reference throughout this specification to features, advantages, orsimilar language does not imply that all of the features and advantagesthat may be realized with the present invention should be or are in anysingle embodiment. Rather, language referring to the features andadvantages is understood to mean that a specific feature, advantage, orcharacteristic described in connection with an embodiment is included inat least one embodiment. Thus, discussions of the features andadvantages, and similar language, throughout this specification may, butdo not necessarily, refer to the same embodiment.

Furthermore, the described features, advantages, and characteristics ofthe invention may be combined in any suitable manner in one or moreembodiments. One skilled in the relevant art will recognize, in light ofthe description herein, that the invention can be practiced without oneor more of the specific features or advantages of a particularembodiment. In other instances, additional features and advantages maybe recognized in certain embodiments that may not be present in allembodiments of the invention.

Reference throughout this specification to “one embodiment,” “anembodiment,” or similar language means that a particular feature,structure, or characteristic described in connection with the indicatedembodiment is included in at least one embodiment. Thus, the phrases “inone embodiment,” “in an embodiment,” and similar language throughoutthis specification may, but do not necessarily, all refer to the sameembodiment.

FIG. 1 is a diagram of a multi-channel radio frequency (RF) transmittersystem 100 in accordance with an embodiment of the invention. Asexplained below, the multi-channel RF transmitter includes mechanisms tocalibrate phase rotators in the transmitter, perform phase alignment andcheck for phase drift to ensure that the phase rotators are performingproperly. Although the multi-channel RF transmitter can be used forvarious application, one application of interest is a radar system forautonomous vehicles.

As shown in FIG. 1, the multi-channel RF transmitter 100 includes an I/Qcoupler module 102, a first channel transmitter module TX1, a secondchannel transmitter module TX2, digital-to-analog converters (DACs) 104,106, 108 and 110, a mixer 112, an analog-to-digital converter (ADC) 114and a fast Fourier transform (FFT) module 116 and a digital processingsystem that includes a phase error controller 118 and a direct digitalsynthesizer (DDS) 120. The multi-channel RF transmitter has an input122, which receives an input signal from a local oscillator (not shown),and outputs 124 and 126, which transmit output signals TX1_OUT andTX2_OUT from the first and second channel transmitter modules TX1 andTX2, respectively.

The I/Q coupler module 102 is connected to the input 122 to receive theinput signal and convert it to a complex (I/Q) RF signal consisting of afirst (I) signal component cos(ω_(LO)*t) and a second (Q) signalcomponent sin(ω_(LO)*t), where ω_(LO) is the frequency of the localoscillator, which are received by the first and second channeltransmitter modules TX1 and TX2.

The first channel transmitter module TX1 includes a phase shifter (alsoknown as a phase rotator) 128 and a power amplifier 130. The phaseshifter receives the complex (I/Q) RF signal from the I/Q coupler module102. The phase shifter 128 also receives an intermediate frequency (IF)signal from the DDS 120 via the DACs 104 and 106, which in the exampleshown in FIG. 1 is dynamically applied to the phase shifter 128 andconsists of a first IF signal component cos(ω_(IF)*t) and a second IFsignal component sin(ω_(IF)*t), where ω_(IF) is the frequency of IFsignal, cos(2πƒ₀*t), where ƒ₀ is in the order of MHz, which can bedefined by the equation

${f_{0} = {\frac{{FS}_{DDS}*{cmd}}{2\hat{}16} = {\frac{FS\_ ADC}{npts\_ fft}*k}}},$

where FS_(DDS) is the sampling frequency of the DDs, cmd is the digitalcontrol of the DDS 120, which is used to program the wanted IFfrequency, FS_ADC is the sampling frequency of the ADC 114, npts_fft isthe number of points used in the discrete Fourier transform, and k is aninteger. In the example shown in FIG. 1, the first and second channeltransmitter modules TX1 and TX2 are both being calibrated at the sametime. The output of the first channel transmitter module TX1 in thisexample is a signal cos(ω_(LO)*t+ω_(IF)*t), which is amplified by thepower amplifier 130, and transmitted to the mixer 112 for calibration.During normal operations, the output of the power amplifier 130 would betransmitted to the output 124.

The second channel transmitter module TX2 includes a phase shifter orrotator 132 and a power amplifier 134. The phase shifter 134 alsoreceives the complex (I/Q) RF signal from the I/Q coupler module 102.The phase shifter 132 also receives a phase code signal from the DDS 120via the DACs 108 and 110, which in the example shown in FIG. 1 isstatically applied to the phase shifter 134 and consists of a firstphase shift signal component cos(ϕ₁) and a second phase shift to signalcomponent sin(ϕ₁), where ϕ₁ is the wanted phase code that is applied onthe phase rotator. The output of the second channel transmitter moduleTX2 is a signal cos(ω_(LO*t)+ϕ₁), which is amplified by the poweramplifier 134, and transmitted to the mixer 112 for calibration. Again,during normal operations, the output of the power amplifier 132 would betransmitted to the output 124.

The mixer 112 is connected to the outputs of the power amplifiers 130and 134 to receive the amplified signals from the first and secondchannel transmitter modules TX1 and TX2. The output of the mixer 112 isconnected to the ADC 114, where mixed analog signal cos(ω_(IF)*t+ϕ₁)from the mixer in the illustrated example is converted to a digitalsignal. The FFT module 116 is connected to the ADC 114 to receive thedigital signal and execute FFT on the digital signal, in an embodiment,the FFT module 116 executes 16 points FFT on the digital signal. As anexample, the FFT module may use a frequency bin of 2.5 MHz for a 40 MHzrange. The output of the FFT module is used by the phase errorcontroller 118, as explained below. The phase error controller 118 canbe any digital signal processing device, such as a microcontroller or adigital processor. The operations performed by the phase errorcontroller 118 will be described below.

The first and second channel transmitter modules TX1 and TX2 provideoutput signals with phase rotations. There are various influences on thephase rotations provided by the first and second channel transmittermodules TX1 and TX2, in particular, the phase shifters or rotators 128and 132, that may cause phase error, as described below.

The first influence of interest is the influence of phase rotator gainimbalance on phase error. This influence is illustrated in FIG. 2A,which illustrates phase errors of +/−9 degrees caused by gain imbalanceof 2.8 dB. With this gain imbalance, the output phase is defined by[cos(LO) cos(ϕ)−(2.8 dB)*sin(LO) sin(ϕ)]. The left graph in FIG. 2A is agraph of phase errors over different phase codes from 0 to 360 degrees,which shows phase errors of +/−9 degrees. The right graph in FIG. 2A isa polar coordinate graph showing points for an ideal signal versuspoints for a signal with phase rotator gain imbalance at the mixer 112of the multi-channel RF transmitter 100.

The second influence of interest is the influence of phase rotator phaseimbalance on phase error. This influence is illustrated in FIG. 2B,which illustrates phase errors of +1.9 degrees maximum caused by phaseimbalance of 3.8 degrees. With this phase imbalance, the output phase isdefined by [cos(LO) cos(ϕ)−sin(LO) sin(ϕ+3.8)]. The left graph in FIG.2B is a graph of phase errors over different phase codes from 0 to 360degrees, which shows phase errors of +3.8/2 degrees maximum. The rightgraph in FIG. 2B is a polar coordinate graph showing points for an idealsignal versus points for a signal with phase rotator phase imbalance atthe mixer 112 of the multi-channel RF transmitter 100.

The third influence of interest is the influence of phase rotatorLO-to-RF leakage on phase error. Phase rotator LO-to-RF leakage is theresultant leakage from the I and Q signals in the phase rotators 128 and132, which involves the leakage from the LO, which is input to each ofthe phase rotators, to the RF signal, which is the output from eachmixer in each of the phase rotators. This influence is illustrated inFIG. 2C, which illustrates phase error of +4 degrees caused by LO-to-RFleakage of the I and Q mixer of the phase rotator, where the leakage forI is 26 dB and the leakage for Q is 26 dB. The left graph in FIG. 2C isa graph of phase errors over different phase codes from 0 to 360degrees, which shows the phase errors of +1.32 degrees to −6.788degrees. The right graph in FIG. 2C is a polar coordinate graph showingpoints for an ideal signal versus points for a signal with phase rotatorLO-to-RF leakage at the mixer 112 of the multi-channel RF transmitter100.

The combined effect of these influences or phase influencing factors isillustrated in FIG. 2D, which illustrates phase error of +/−11 degreescaused by gain imbalance of 2.8 dB, phase imbalance of 3.8 degrees andLO-to-RF leakage of 23 dB. The left graph in FIG. 2D is a graph of phaseerrors over different phase codes from 0 to 360 degrees, which showsphase errors of +10 degrees to −13 degrees. The right graph in FIG. 2Dis a polar coordinate graph showing points for an ideal signal versuspoints for a signal with phase rotator gain imbalance, phase imbalanceand LO-to-RF leakage at the mixer 112 of the multi-channel RFtransmitter 100.

In order to calibrate the phase shifters 128 and 132 of the first andsecond channel transmitter modules TX1 and TX2 to reduce theseinfluences, the multi-channel RF transmitter 100 is designed to performthree successive calibrations, which may be done in any order, in whatwill be referred to herein as a three-step calibration process. Thesethree calibrations include (1) calibration of LO-to-RF leakage, (2)calibration of phase rotator gain imbalance and (3) calibration of phaserotator phase imbalance. These calibrations involve selectingcompensation values to adjust or modify the signals from the DDS 120 tothe phase shifters 128 and 132 to compensate for these influences. Thisis illustrated in FIG. 3, which shows a modified IF signal that isapplied to the phase rotator 128 of the first second channel transmittermodule TX1 and a modified phase code signal that is applied to the phaserotator 132 of the second channel transmitter module TX2 (DACs 104, 106,108 and 110 are not illustrated in this figure). As shown in FIG. 3, themodified signals include a DC offset value Offset_predist to compensatefor phase rotator LO-to-RF leakage, a phase value ϕ_predist tocompensate for phase rotator phase imbalance, and a gain value G_predistto compensate for phase rotator gain imbalance. The basis for usingthese values on the signals on the phase rotators is illustrated inFIGS. 4A and 4B.

FIG. 4A shows the phase rotator 128 of the first channel transmittermodule TX1 that receives the complex (I/Q) RF signal consisting of (I)signal component cos(ω_(LO)*t) and (Q) signal component sin(ω_(LO)*t)from the coupler module 102. In addition, the phase rotator 128 receivedan IF signal consisting of a phase shift signal component cos(ω_(IF)*t)and a phase shift signal component sin(ω_(IF)*t). In an ideal case, theoutput of the phase rotator would be cos(LO) cos(ϕ)−sin(ϕ) sin(LO).However, if there is phase and gain imbalances, the actual output of thephase rotator 128 would be cos(LO) cos(ϕ)−G _imb*sin(ϕ) sin(LO+Δϕ_imb),where G_imb is the gain imbalance and Δϕ_imb is the phase imbalance, asillustrated in FIG. 4A. Thus, the actual output of the phase rotator 128would include phase error with respect to the ideal case. This phaseerror can be theoretically canceled by modifying the IF signal appliedto the phase rotator 128.

As illustrated in FIG. 4B, the IF signal applied to the phase rotator128 has been modified to a signal consisting of a phase shift signalcomponent cos(ω_(IF)*t) and a second phase shift signalG_predist*sin(ω_(IF)*t+ϕ_predist), where G_(predist)=1/G_imb andΔϕ_predist=Δϕ_imb. Using this modified IF signal, the output of thephase rotator would be cos(LO) cos(ϕ)−G _imb*G_predist*sin(ϕ+Δϕ_predist)sin(LO+Δϕ_imb) so that the phase error caused by the gain imbalance andthe phase imbalance would be canceled out.

The three-step calibration process performed by the multi-channel RFtransmitter 100 in accordance with an embodiment is described withreference to a process flow diagram of FIGS. 5A, 5B and 5C. In thisembodiment, the three-step calibration process involves the followingorder of performing calibrations: LO-to-RF leakage calibration (FIG.5A), gain imbalance calibration (FIG. 5B) and phase imbalancecalibration (FIG. 5C). However, as previously mentioned, these threecalibrations may be performed in any order, such as gain imbalancecalibration, LO-to-RF leakage calibration and then phase imbalancecalibration.

As shown in FIG. 5A, the three-step calibration process begins at block502, where a first LO-to-RF leakage compensation value (“DC offsetvalue”) is selected. A LO-to-RF leakage compensation value is a value toproduce an opposite offset of an offset caused by LO-to-RF leakage. Inan embodiment, the LO-to-RF leakage compensation value may represent anoffset value in mV. Next, at block 504, a phase code corresponding to afirst phase is selected. Next, at block 506, an IF signal with theselected LO-to-RF leakage compensation value is applied to the phaseshifter 128 of the first channel transmitter module TX1. Next, at block508, the phase code with the same selected LO-to-RF leakage compensationvalue is applied to the phase shifter 132 of the second channeltransmitter module TX2. Next, at block 510, the resultant phase errordue to the phase errors in the outputs of the phase shifters 128 and 132is computed using the FFT results from the FFT module 116 and stored ina storage accessible by the phase error controller 118. Next, at block512, a determination is made whether the current phase code correspondsto the last phase code to be used. If the current phase code is not thelast phase code for this iteration, then the process proceeds back toblock 504, where the next phase code is selected to be applied to thephase shifter 132 of the second channel transmitter module TX2, whilethe IF signal is applied to the phase shifter 128 of the first channeltransmitter module TX1, to compute another resultant phase error due tothe phase errors in the outputs of the phase shifters 128 and 132 of thefirst and second channel transmitter modules TX1 and TX2 for theselected phase code. In an embodiment, the phase codes correspond to 0to 360 degrees in fixed steps, e.g., 45 degrees. Thus, in thisembodiment, there will be eight phase codes that need to be used. If thecurrent phase code is the last phase code for this iteration, then theprocess proceeds to block 514.

At block 514, a determination is made whether the current LO-to-RFleakage compensation value is the last compensation value to be used. Ifthe current LO-to-RF leakage compensation value is not the last value,then the process proceeds back to block 502, where the next LO-to-RFleakage compensation value is selected to be used. In an embodiment, theLO-to-RF leakage compensation values correspond to 20 dB to 45 dB inpredefined increments, e.g., 1 or 5 dB, which can be achieved byincrementing the least significant bit (LSB) of the DACs 104, 106, 108and 110. However, if the current LO-to-RF leakage compensation value isthe last value, then the process proceeds to block 516.

At block 516, the phase error controller 118 sums the resultant phaseerrors associated with the different phase codes for each of theLO-to-RF leakage compensation values. Next, at block 518, the phaseerror controller 118 selects the LO-to-RF leakage compensation valuewith the minimum summed resultant phase error value to be used as theLO-to-RF leakage compensation value for the other calibrations. Thus,the optimal LO-to-RF leakage compensation value has been selected forthe phase shifters 128 and 132 of the first and second channeltransmitter modules TX1 and TX2.

As shown in FIG. 5B, the gain imbalance calibration of the three-stepcalibration process begins at block 520, where a first gain imbalancecompensation value is selected. Next, at block 522, a phase codecorresponding to a first phase is selected. Next, at block 524, an IFsignal with the optimal LO-to-RF leakage compensation value and theselected gain imbalance compensation value is applied to the phaseshifter 128 of the first channel transmitter module TX1. Next, at block526, the phase code with the optimal LO-to-RF leakage compensation valueand the same selected gain imbalance compensation value is applied tothe phase shifter 132 of the second channel transmitter module TX2.Next, at block 528, the resultant phase error due to the phase errors inthe outputs of the phase shifters 128 and 132 of the first and secondchannel transmitter modules TX1 and TX2 is computed using the FFTresults from the FFT module 116 and stored in a storage accessible bythe phase error controller 118. Next, at block 530, a determination ismade whether the current phase code corresponds to the last phase codeto be used. If the current phase code is not the last phase code forthis iteration, then the process proceeds back to block 522, where thenext phase code is selected to be applied to the phase shifter 132 ofthe second channel transmitter module TX2, while the IF signal isapplied to the phase shifter 128 of the first channel transmitter moduleTX1, to compute another resultant phase error due to the phase errors inthe outputs of the phase shifters 128 and 132 of the first and secondchannel transmitter modules TX1 and TX2 for the selected phase code. Inan embodiment, the phase codes correspond to 0 to 360 degrees in fixedsteps, e.g., 45 degrees. Thus, in this embodiment, there will be eightphase codes that need to be used. If the current phase code is the lastphase code for this iteration, then the process proceeds to block 532.

At block 532, a determination is made whether the current gain imbalancecompensation value is the last value to be used. If the current gainimbalance compensation value is not the last value, then the processproceeds back to block 520, where the next gain imbalance compensationvalue is selected to be used. In an embodiment, the gain imbalancecompensation values correspond to −4 dB to +4 dB in predefinedincrements, e.g., 0.1 dB. However, if the current gain imbalancecompensation value is the last value, then the process proceeds to block530.

At block 534, the phase error controller 118 sums the resultant phaseerrors associated with different phase codes for each of the gainimbalance compensation values, Next, at block 536, the phase errorcontroller 118 selects the gain compensation value with the minimumsummed resultant phase error value to be used as the gain imbalancecompensation value for the remaining calibration. Thus, the optimal gainimbalance compensation value has been selected for the phase shifters128 and 132 of the first and second channel transmitter modules TX1 andTX2, in addition to the optimal LO-to-RF leakage compensation value.

As shown in FIG. 5C, the phase imbalance calibration of the three-stepcalibration process begins at block 538, where a first phase imbalancecompensation value is selected. Next, at block 540, a phase codecorresponding to a first phase is selected. Next, at block 542, an IFsignal with the optimal LO-to-RF leakage compensation value, the optimalgain imbalance compensation value and the selected phase imbalancecompensation value is applied to the phase shifter 128 of the firstchannel transmitter module TX1. Next, at block 544, the phase code withthe optimal LO-to-RF leakage compensation value, the optimal gainimbalance compensation value and the selected phase imbalancecompensation value is applied to the phase shifter 132 of the secondchannel transmitter module TX2. Next, at block 546, the resultant phaseerror due to the phase errors in the outputs of the phase shifters 128and 132 of the first and second channel transmitter modules TX1 and TX2is computed using the FFT results from the FFT module 116 and stored ina storage accessible by the phase error controller 118. Next, at block548, a determination is made whether the current phase code correspondsto the last phase code to be used. If the current phase code is not thelast phase code for this iteration, then the process proceeds back toblock 540, where the next phase code is selected to be applied to thephase shifter 132 of the second channel transmitter module TX2, whilethe IF signal is applied to the phase shifter 128 of the first channeltransmitter module TX1, to compute the phase error in the outputs of thephase shifters 128 and 132 of the first and second channel transmittermodules TX1 and TX2 for the selected phase code. In an embodiment, thephase codes correspond to 0 to 360 degrees in fixed steps, e.g., 45degrees. Thus, in this embodiment, there will be eight phase codes thatneed to be used. If the current phase code is the last phase code forthis iteration, then the process proceeds to block 550.

At block 550, a determination is made whether the current phaseimbalance compensation value is the last value to be used. If thecurrent phase imbalance compensation value is not the last value, thenthe process proceeds back to block 538, where the next phase imbalancecompensation value is selected to be used. In an embodiment, the phasecompensation values correspond to −5 degrees to +5 degrees in predefinedincrements, e.g., 0.2 degrees. However, if the current phase imbalancecompensation value is the last value, then the process proceeds to block552.

At block 552, the phase error controller 118 sums the resultant phaseerrors associated with different phase codes for each of the phaseimbalance compensation values. Next, at block 554, the phase errorcontroller 118 selects the phase imbalance compensation value with theminimum summed resultant phase error value. Thus, the optimal phaseimbalance compensation value has been selected for the phase shifters128 and 132 of the first and second channel transmitter modules TX1 andTX2, in addition to the optimal LO-to-RF leakage and gain imbalancecompensation values. These compensation values can then be used tomodify the signals from the DDS 120 used on the phase shifters 128 and132 of the first and second channel transmitter modules TX1 and TX2 toensure that their outputs are the desired signals with respect to phaseand amplitude.

The different phase errors that are captured by the phase errorcontroller 118 for the different compensation values during each of thethree calibrations are illustrated in FIG. 6, which is a graph showingphase errors for different phase compensation values during the phaseimbalance calibration. In this graph, each line represents the phaseerrors for a particular phase compensation value.

In simulations, for gain imbalance=2.4 dB, phase imbalance=3.3 degreesand LO-to-RF leakage =23 dB, the phase error is +/−10 degrees before thethree-step calibration process depicted in FIGS. 5A-5C. After thethree-step calibration process, the phase error is less than or equal to0.5 degrees.

The results of the three-step calibration process can depend on theresolution of the DACs 104. 106, 108 and 110, and thus, can be reducedby using different types of DACs. This is illustrated in FIGS. 7A, 7Band 7C. FIG. 7A shows the phase errors due to gain imbalance for nocalibration, for calibration using 8-bit DACs and for calibration using15-bit DACs. FIG. 7B shows the phase errors due to phase imbalance forno calibration, for calibration using 8-bit DACs and for calibrationusing 15-bit DACs. FIG. 7C shows the phase errors due to LO-to-RFleakage for no calibration, for calibration using 8-bit DACs and forcalibration using 15-bit DACs.

The described three-step calibration process calibrates both the firstand second transistor modules TX1 and TX2. The same three-stepcalibration process can be applied with the first and second transistormodules TX1 and TX2 reversed. The results would be similar since thefirst and second transistor modules TX1 and TX2 are similar. Althoughthe three-step calibration process has been described using two channeltransmitter modules, the described three-step calibration process may beextended for three channel transmitter modules, which may simply involvedifferent connections between the three channel transmitter modules, theDDS 120 and the mixer 112 so that only two of the three transmittermodules are involved for each three-step calibration process.

After the three-step calibration process, a phase alignment processshould be performed to phase align between the first and second channeltransmitter modules TX1 and TX2. This phase alignment process will bedescribed with reference to FIG. 8. In this phase alignment process, thesame phase code (phase code 0) with the same predistortion orcompensation is applied on both the first and second channel transmittermodules TX1 and TX2 from the DDS 120. In addition, a complex (I/Q) RFsignal consisting of a first (I) signal component cos(ω_(LO)*t) and asecond (Q) signal component sin(ω_(LO)*t) is applied to both the firstand second channel transmitter modules TX1 and TX2 from the I/Q couplerdevice 102. The outputs from the first and second channel transmittermodules TX1 and TX2 will be cos(LO+∅₁) and cos(LO+∅₂), respectively.Thus, the output of the ADC 114 will be cos(∅₂−∅₁). The phase errorcontroller 118 receives the signal from the ADC 114 without FFT andextracts (∅₇−∅₁) using the arccosine function to determine the phasemisalignment between the first and second channel transmitter modulesTX1 and TX2. In an alternative embodiment, the phase difference (∅₂−∅₁)can be extracted by applying FFT to the received signal from the ADC 114and using the arctangent function. Using this information, the phaseerror controller 118 provides a phase offset to the DDS 120 tocompensate for the phase misalignment so that the DDS modifies signalsto the first and second transmitter modules TX1 and TX2 so that ∅₂−∅₁ iszero or close to zero (e.g., <1 degrees) to phase align the first andsecond channel transmitter modules TX1 and TX2.

As the multi-channel RF transmitter 100 operates, one or both of thefirst and second channel transmitter modules TX1 and TX2 may experiencephase error drift due to various factors, which may pose a safety threatfor certain applications, such as autonomous driving or collisionavoidance. Thus, the multi-channel RF transmitter may periodicallyperform a safety check on each of the first and second channeltransmitters.

In an embodiment, a safety check of the multi-channel RF transmitter 100involves checking periodically, e.g., daily, weekly, monthly or otherappropriate predefined intervals, the phase error level whencompensation has been applied. After compensation for the first time fora particular phase code, the phase error is measured, and the value ofthe phase error for the phase code is loaded in a register with aprogrammed tolerance to be used as a reference phase error. In case of aproblem in one or both of the first and second channel transmittermodules TX1 and TX2, the phase errors for different phase codes willstart to drift, which can be measured as phase differences, or deltaphases, between the current phase errors and the reference phase errors.If any of the phase differences due to the drift is higher than a firstprogrammable threshold, then a first flag is generated. If the sum ofthe phase differences is greater than a second programmable threshold, asecond flag is generated. The flags are sent to a microcontroller unit,so that an appropriate action can be initiated in response to the flags,e.g., disabling the multi-channel RF transmitter for safety concerns.

FIG. 9 is a process flow diagram of a method of calibrating amulti-channel RF transmitter in accordance with an embodiment of theinvention. At block 902, an IF signal with different compensation valuesis applied on a first phase rotator in a first channel transmittermodule of the multi-channel RF transmitter. The different compensationvalues are designed to compensate for a particular phase influencingfactor, such as a LO-to-RF leakage, a gain imbalance or a phaseimbalance. At block 904, phase codes with the same differentcompensation values for different phases are applied on a second phaserotator in a second channel transmitter module of the multi-channel RFtransmitter. At block 906, resultant phase errors due to phase errors ofthe first and second channel modules for the different compensationvalues are measured. At block 908, based on the resultant phase errors,one of the different compensation values is selected to be used as acalibrated compensation value for the first phase rotator in the firstchannel transmitter module and for the second phase rotator in thesecond channel transmitter module to compensate for the particular phaseinfluencing factor.

Although the operations of the method herein are shown and described ina particular order, the order of the operations of the method may bealtered so that certain operations may be performed in an inverse orderor so that certain operations may be performed, at least in part,concurrently with other operations. In another embodiment, instructionsor sub-operations of distinct operations may be implemented in anintermittent and/or alternating manner.

In addition, although specific embodiments of the invention that havebeen described or depicted include several components described ordepicted herein, other embodiments of the invention may include fewer ormore components to implement less or more feature.

Furthermore, although specific embodiments of the invention have beendescribed and depicted, the invention is not to be limited to thespecific forms or arrangements of parts so described and depicted. Thescope of the invention is to be defined by the claims appended heretoand their equivalents.

1. A method of calibrating a multi-channel radio frequency (RF)transmitter, the method comprising: Applying an intermediate frequency(IF) signal with different compensation values on a first phase rotatorin a first channel transmitter module of the multi-channel RFtransmitter, wherein the different compensation values are designed tocompensate for a particular phase influencing factor; applying phasecodes with the same different compensation values for different phaseson a second phase rotator in a second channel transmitter module of themulti-channel RF transmitter; measuring resultant phase errors due tophase errors of the first and second channel transmitter modules for thedifferent compensation values; and based on the resultant phase errors,selecting one of the different compensation values to be used as acalibrated compensation value for the first phase rotator in the firstchannel transmitter module and for the second phase rotator in thesecond channel module to compensate for the particular phase influencingfactor.
 2. The method of claim 1, wherein the different compensationvalues include DC offset values to compensate for local oscillator to RFleakage, gain values to compensate for gain imbalance, or phase valuesto compensate for phase imbalance.
 3. The method of claim 1, furthercomprising applying a complex I/Q signal to the first and second phaserotators from an I/Q coupler module that receives a local oscillatorsignal.
 4. The method of claim 1, wherein measuring the resultant phaseerror includes mixing the outputs of the first and second channeltransmitter modules to generate a mixed signal and performing fastFourier transform (FFT) on the mixed signal to measure the resultantphase error.
 5. The method of claim 4, wherein measuring the resultantphase error further includes using arctangent function on results of theFFT to measure the resultant phase error.
 6. The method of claim 1,wherein selecting one of the different compensation values includessumming resultant phase errors for the different compensation values toproduce summed resultant phase error values corresponding to thedifferent compensation values and selecting the minimum value from thesummed resultant phase error values to select the calibratedcompensation value.
 7. The method of claim 1, further comprising:applying a particular phase code with the calibrated compensation valueon the first phase rotator in the first channel transmitter module andthe second phase rotator in the second channel transmitter module toproduce output signals from the first and second channel transmittermodules; determining a phase misalignment between the first and secondchannel transmitter modules using the output signals; and providing aphase offset to compensate for the phase misalignment so that signals tothe first and second channel transmitter modules are modified to phasealign the first and second channel transmitter modules.
 8. The method ofclaim 1, further comprising: applying the signal with the selectedcompensation value on the first phase rotator in the first channeltransmitter module; applying the phase codes with the selectedcompensation value on the second phase rotator in the second channeltransmitter module; measuring phase differences between outputs of thefirst and second channel transmitter modules; comparing the phasedifference to at least one programmed threshold; and generating a flagto indicate phase drift when at least one of the phase differences isgreater than the at least one programmed threshold.
 9. A multi-channelradio frequency (RF) transmitter comprising: a first channel transmittermodule comprising a first phase rotator; a second channel transmittermodule comprising a second phase rotator; a mixer to combine outputs ofthe first and second channel transmitter modules; and a digitalprocessing system operatively connected to the first and second channeltransmitter modules and the mixer, the digital processing system beingconfigured to: apply an intermediate frequency (IF) signal withdifferent compensation values on the first phase rotator, wherein thedifferent compensation values are designed to compensate for aparticular phase influencing factor; apply phase codes with the samedifferent compensation values for different phases on the second phaserotator; measure resultant phase errors due to phase errors of the firstand second channel modules for the different compensation values; andbased on the resultant phase errors, select one of the differentcompensation values to be used as a calibrated compensation value forthe first phase rotator in the first channel transmitter module and forthe second phase rotator in the second channel transmitter module tocompensate for the particular phase influencing factor
 10. Thetransmitter of claim 9, wherein the different compensation valuesinclude DC offset values to compensate for local oscillator to RFleakage, gain values to compensate for gain imbalance, or phase valuesto compensate for phase imbalance.
 11. The transmitter of claim 9,wherein the digital processing system is further configured to apply acomplex I/Q signal to the first and second phase rotators from an I/Qcoupler module that receives a local oscillator signal.
 12. Thetransmitter of claim 9 further includes an FFT module to perform FFT ona mixed signal from the mixer for a phase error measurement.
 13. Thetransmitter of claim 12, wherein the digital processing system isfurther configured to use arctangent function on results of the FFT tomeasure the resultant phase error.
 14. The transmitter of claim 9,wherein the digital processing system is further configured to sum theresultant phase errors for the different compensation values to producesummed resultant phase error values corresponding to the differentcompensation values and select the minimum value from the summedresultant phase error values to select the calibrated compensationvalue.
 15. The transmitter of claim 9, wherein the digital processingsystem is further configured to: apply a particular phase code with thecalibrated compensation value on the first phase rotator n the firstchannel transmitter module and the second phase rotator in the secondchannel transmitter module to produce output signals from the first andsecond channel transmitter modules; determine a phase misalignmentbetween the first and second channel transmitter modules using theoutput signals; and provide a phase offset to compensate for the phasemisalignment so that signals to the first and second channel transmittermodules are modified to phase align the first and second channeltransmitter modules.
 16. The transmitter of claim 9, wherein the digitalprocessing system is further configured to apply the signal with theselected compensation value on the first phase rotator in the firstchannel transmitter module.
 17. The transmitter of claim 9, wherein thedigital processing system is further configured to apply the phase codeswith the selected compensation value on the second phase rotator in thesecond channel transmitter module.
 18. The transmitter of claim 9,wherein the digital processing system is further configured to measurephase differences between outputs of the first and second channeltransmitter modules.
 19. The transmitter of claim 18, wherein thedigital processing system is further configured to compare the phasedifference to at least one programmed threshold.
 20. The transmitter ofclaim 20, wherein the digital processing system is further configured togenerate a flag to indicate phase drift when at least one of the phasedifferences is greater than the at least one programmed threshold.